Doors of the latter type have been in use in recent years in passenger vehicles because this construction provides a better protection of the passenger compartment against lateral impact or collision than does a door in which the threshold merely underlies the frame of the swingable door in the closed position thereof and also because the door can prevent soiling of the threshold and protect it from minor damage which might affect a threshold underlying the door.
For a variety of passenger automobiles and especially European-constructed passenger automobiles intended for sale in the United States, it is common to provide a door reinforcing beam in the vehicle door which extends horizontally at bumper height to provide impact resistance in the case of a lateral impact.
In accidents in which the collision results in an impact below such reinforcing beams, the reinforcement element is of little help and is not capable of providing significant protection. The collision force readily deforms the outer skin of the door inwardly since the skin provides little energy dissipation or resistance to such impact. The door frame is readily deformed and, if the threshold member is laterally juxtaposed with the lower portion of the frame, the threshold member can be deformed as well.
The entire system provides little deformation resistance or energy dissipation although the vehicle body offers ample opportunity to dissipate some of this energy by being able to take up some of the impact energy and a substantial vehicle body strength is available at the threshold member. In other words, the vehicle body does not provide, for such impacts, a deformation resistance capable of utilizing the girder and beam strengths of the vehicle body. If, of course, the full deformation thereof can be brought into play with such collisions, a greater ability to protect passenger and driver can be gained.
Of course, that requires that the deformation energy be taken up in the door and that the door provide an ample deformation resistance, indeed a deformation resistance which is sufficient to dissipate the energy of impact to a degree that the beams and girders of the passenger compartment will not be plastically deformed.